This invention relates to well drilling operations and more particularly to a system for accurately determining the depth of the drilling tool in the well.
Drilling of oil wells has progressed from crude drilling rigs, to cable tool rigs, to the modern rotary drilling rigs. In rotary drilling, a power rotating means delivers torque to a drill pipe which turns a bit drilling a borehole into the subsurface formations. The drill pipe is lowered and raised in the borehole by means of a support cable extending from a suitable drawworks drum. The depth of the bit in the borehole has, in the past, been measured by detecting movement of the support cable and then relating this movement to drill bit depth.